Remediation technologies are selected for the site based on site geology, hydrogeology, chemicals of concern, efficiency of selected removal rates, and cost effectiveness of the removal rates for the specific site. Some of the remediation methods available for sites are listed below.
Soil Vapor Extraction
Soil vapor extraction (SVE) is an efficient remediation technology used to extract hydrocarbon vapors from relatively permeable, unsaturated soil zones. When relatively light LPH is present (such as gasoline, chlorinated compounds, etc.), the SVE technology promotes some volatilization of the product from the subsurface. Further, SVE blowers can be used to apply a vacuum to the wellheads to enhance the ground water flow rate to the wells. One of the goals with SVE systems, to extract hydrocarbon vapors via airflow and enhance the soil cleanup, is likely achieved through implementation. SVE systems are low maintenance and easy to operate. This technology can also be very useful for supplying oxygen to support indigenous biological growth for bioremediation (also called bioventing). Bioventing can be an additional method for petroleum hydrocarbon removal from the soil. Further, if the soil is relatively homogeneous, application of SVE produces a large Radius of Influence (ROI), channeling is minimized, and extraction efficiency is maximized.
Air SpargingAir sparging is most effective in porous soils and would have limited effectiveness in sites consisting of more clayey soils. Air sparging is not appropriate for sites with low benzene concentrations in ground water. Air sparging utilizes an air pressure pump to introduce air into the subsurface. The forced air volatilizes the petroleum hydrocarbons in the ground water and adsorbed onto the soil particles in the subsurface. Air sparging ismost effective in porous soils but has demonstrated some effectiveness in moderately stiff clay materials as well.
Monitored Natural Attenuation
The processes of biodegradation, volatilization, dispersion, and hydrolysis act to naturally attenuate concentrations of hydrocarbons, dissolved in ground water at sites. The use of demonstrated natural attenuation as a corrective action at sites requires directly or indirectly documenting these processes. There are several ways to document the occurrence of natural attenuation of hydrocarbons. The first involves using historical trends in contaminant concentrations to show that a reduction in the total mass of contaminants is occurring at the site. The second involves documenting that the electron acceptor concentration in up gradient ground water is enough to facilitate degradation of dissolved contaminants.
Soil Excavation and Disposal
This option is generally efficient and cost effective for removing source materials and/or impermeable materials that will not be addressed via the induction of air flow in the subsurface. Over excavation of soils and then ground water monitoring for two or four quarters may indicate that installation of a chemical or mechanical remediation system may not be necessary.
Air Sparging
The in-situ Air Sparging technology is generally effective and cost efficient but requires relatively permeable, homogeneous soils to allow movement of air through the saturated zone. The Air Sparging process strips the ground water of the Volatile Organic Compounds (VOCs) very efficiently if the impacted ground water plume is targeted well. However, SVOCs in the ground water are generally unaffected by this volatilization process. The SVOCs present in the ground water can be removed through one of these more efficient methods: chemical oxidation, bioremediation, or some type of chemical adsorption. In addition, if liquid phase hydrocarbon (LPH) is present, the turbulence caused in ground water by air sparging can mix the LPH with the ground water and potentially cause an increase in the concentration of the dissolved compounds. Additionally, air sparging can force LPH into areas not previously impacted by LPH.
Multi Phase Extraction High Vacuum Multi Phase Extraction (MPE): MPE technology utilizes a common well to extract petroleum vapors, groundwater and free product from an extraction well. Two or four inch diameter extraction wells are typically used. A one-inch diameter open-ended pipe (stinger) is placed in each well to the desired depth of groundwater drawdown. The pipe is connected to a vacuum pump through a pipe network. After the groundwater has been drawn down to the desired depth, the vacuum pump continues to extract water from the wells at the recharge rate of the water-bearing unit.
Additionally, petroleum vapors can be extracted while groundwater is not available. The groundwater is removed from the air stream prior to entering the vacuum pump by using a large diameter vessel (knockout tank) designed to allow water to "drop" out of the air stream. The vapors are discharged through the vacuum pump while the water is pumped through the treatment equipment. The vapors are discharged to the atmosphere or are treated to remove contaminants if the volatile organic compound discharge concentrations exceed regulatory limits. Unlike some pump and treat systems, the MPE system equipment is placed in a treatment building, not down wells, which makes the operation and maintenance of the unit relatively easy. Further, due to the high vacuums created by these systems, LPH present on the ground water and soil volatilizes and can be extracted and vented to the atmosphere. Hydrocarbon volatilization due to high vacuum systems is a major source of LPH removal, which can be an advantage in that the removal of free product is expedited. High vacuum pumps are typically more efficient in simultaneously extracting groundwater and hydrocarbon vapors from the soil to achieve the desired remediation results. Additionally, the stinger can be placed at a depth just below the LPH/water interface for controlled product removal.
Air Sampling – Emissions
Vapor samples are collected approximately 30 minutes after the initiation of each pilot test and submitted for BTEX and C4-C18 range petroleum hydrocarbons air analysis. The purpose of the air analysis is to obtain data to calculate air emissions estimates and determine air permitting requirements and contaminant mass removal.